1. Field of the Invention
This invention relates to drilling assemblies, apparatus and methods utilizing a drilling motor and stabilizer and more particularly, but not by way of limitation, to techniques for reducing "hang-up" and "whirl" during the drilling of a borehole in an earthen formation.
2. Setting of the Invention
One technique for drilling a substantially vertical borehole in an earthen formation, such as done during prospecting for oil and gas, includes rotating a drill string with surface equipment and independently rotating a drill bit with a drilling motor located near the bottom of the drill string above the drill bit. Bit advancement or penetration is obtained, at least in part, by controlling the application of weight on the bit. A stabilizer apparatus is typically used to control to some extent the direction of drilling by reducing lateral movement or "whirl" of the drill bit as it is rotated at high speeds by the downhole drilling motor (see, U.S. Pat. Nos. 4,485,879 to Kamp, et al.; 4,643,264 to Dellinger; and 4,364,626 to Derouin).
A stabilizer can be mounted on a downhole drilling motor as disclosed in the aforementioned Kamp, et al., patent. A stabilizer can have a cutting or reaming capability as disclosed in U.S. Pat. Nos. 4,618,010 to Falgbility, Sr., et al.; 4,385,669 to Knutsen; 4,456,080 to Holbert; and 4,373,592 to Dellinger, et al. A stabilizer can be wider than a drill bit or radially extendible as disclosed in U.S. Pat. No. 4,577,701 to Dellinger, et al.; Juge, "New Bit Aids Deviated Well Drilling," Drilling Contractor (1979); U.S. Pat. Nos. 4,690,229 to Raney; and 4,270,619 to Base.
There is also a disclosure about a generally cone-shaped combination reamer and stabilizer tool disclosed to be connected above a drill bit and rotated therewith to form a curved borehole. The drill bit is pictured as having a smaller diameter than the base of the combination reamer and stabilizer tool (see U.S. Pat. No. 4,523,652 to Schuh).
Although a stabilizer can be used to maintain a proper drilling direction, or at least to reduce bit "whirl," it can "hang-up" on the uneven surface of the open borehole as the borehole is being drilling. Such "hang-up" can occur to such an extent that it prevents smooth control of the application of weight-on-bit needed to obtain a desired bit advancement or rate of penetration. Such hang-up has been found to occur particularly with respect to a drilling assembly incorporating a downhole drilling motor having a stabilizer with an overall outer diameter less than or equal to the drill bit diameter. Thus, there is the need for a better technique for reducing whirl while also reducing the likelihood of "hang-up." It is contemplated that such an improved technique would produce better, more consistent control of the application of weight to the drill bit and thus better, more consistent control of the rate of penetration. Improving control of the rate of penetration can reduce drilling costs.